Renames PK/reference fields across chat models, services, serializers,
views, urls, websocket routing, and events (id->uuid, user_id->user_uuid,
chat_id->chat_uuid, sender_id->sender_uuid, user_1_id/user_2_id->
user_1_uuid/user_2_uuid) for consistency with the uuid-named PK
convention already used in core/users/gooyal_oauth2. Mattermost's own
opaque identifiers (post_id, channel_id, mattermost_user_id) are left
unchanged since they are not UUIDs.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
